The present invention relates to microporous materials, including microporous materials formed by crystallization of a propylene-containing polymer in the presence of a beta-nucleating agent. The invention also relates to methods of forming microporous materials and articles made using the microporous materials.
Microporous materials are used in a wide range of applications, including filters, breathable membranes, and absorbent articles. Numerous methods exist for making microporous articles. However, a need exists for improved microporous materials and methods of making the microporous materials.
The present invention is directed to microporous materials suitable for use in a wide range of applications, such as oil and water-absorbing films, battery separators, and diffusion barriers. The microporous materials contain a combination of a crystallizable propylene-containing polymer and a beta-nucleating agent. A diluent material, present during formation of the microporous materials, is also typically present in the microporous materials. The diluent material is miscible with the polymer at a temperature above the melting point of the polymer, yet phase separates from the polymer at a temperature below the polymer crystallization temperature. The crystallizable propylene-containing polymer can be polypropylene, a propylene-containing copolymer, or a mixture of thermoplastic polymers including polypropylene. The diluent is, for example, mineral oil. The beta-nucleating agent can be, for example, quinacridone colorants (such as gamma quinacridone), dicarboxylic acid salts of metals of Group IIA of the periodic table.
The microporous materials of the invention are formed using a thermally induced phase separation (TIPS) method. This method of making the microporous materials typically includes melt blending the crystallizable propylene-containing polymer, the beta-nucleating agent, and the diluent to form a melt mixture. After creating this melt mixture it is formed into a shaped article and cooled to a temperature at which the beta-nucleating agent nucleates crystals of the propylene-containing polymer and causes phase separation between the polymer and diluent. In this manner a material is formed that comprises an aggregate of a plurality of crystallized propylene-containing polymer particles in the diluent compound.
Following formation of the particles of the polymer, the porosity of the material is increased by one or both of stretching the shaped article in at least one direction and removing at least a part of the diluent. This step results in separation of adjacent particles of propylene-containing polymer from one another to provide a network of interconnected micropores. This step also permanently attenuates the propylene-containing polymer to form fibrils connecting the plurality of particles, imparting strength and porosity to the article. The diluent compound can be removed from the material either before or after stretching. In certain embodiments, the diluent compound is not removed.
The use of beta-nucleating agents in accordance with the present invention permits the manufacture of a wide variety of microporous materials previously not possible. They permit microporous films to be made, and crystallized, over a higher and broader range of temperatures than are normally accessible. These conditions can produce films having different characteristics than films made at lower temperatures. In addition, according to the present invention, only small amounts of the beta-nucleating agents are needed to make microporous materials. Accordingly, materials having favorable color properties, including a lack of color, can be formed using the beta-nucleating agents of the invention because the color of the beta-nucleating agent does not interfere with the desired color due to its low concentration.
The polymer particles form in the presence of a beta-nucleating agent such that the number of fibrils connecting the particles per unit volume can be significantly increased over the number of fibrils that would exist per unit volume if no beta-nucleating agent were present. If no beta-nucleating agent were present, fewer and larger spherulites would slowly form, thus providing fewer fibrils resulting in a film with little or no strength. In the present invention, as the melt mixture cools in the presence of a beta-nucleating agent, the polymer phase separates quickly from the diluent and forms more and smaller spherulites, and more fibrils. The fibril lengths in the microporous article are typically increased and the tensile strength enhanced, and the microporous material typically has significantly improved stretchability compared to forming the microporous material without a beta-nucleating agent.
Other features and advantages of the invention will be apparent from the following detailed description of the invention and the claims. The above summary of principles of the disclosure is not intended to describe each illustrated embodiment or every implementation of the present disclosure. The detailed description that follows more particularly exemplifies certain embodiments utilizing the principles disclosed herein.